The present invention relates to a fuel injection control apparatus for an engine, particularly to a fuel injection control apparatus for an engine which injects fuel into a cylinder a plurality of times to cause a plurality of combustions therein.
Conventionally, various kinds of measures are conducted to reduce noises in diesel engines, particularly the noise caused by knocking in the engine (hereinafter, simply referred to as “knocking noise”). For example, JP3803903B2 discloses an art of calculating a target value of a produced time difference between combustion pressure waves which are produced by a plurality of fuel injections, respectively, and controlling a time interval of the plurality of fuel injections based on the target value. A time difference with which the combustion pressure waves interfere with each other and a pressure level can be lowered within a high frequency band is calculated as the target value. With this art, by controlling the time interval of the fuel injections, a frequency component of pressure inside a cylinder (in-cylinder pressure) is reduced by targeting a specific frequency band (2.8 to 3.5 kHz), so as to reduce the knocking noise. Note that the phrase “combustion pressure wave” indicates a pressure wave produced when the in-cylinder pressure dramatically increases due to the combustion inside the engine, and corresponds to a result of differentiating the waveform of the in-cylinder pressure by time (similar for below).
The knocking noise caused in the engine has transmission characteristics of a structural system of the engine, particularly characteristics corresponding to resonant frequencies of the structural system of the engine. Specifically, the knocking noise tends to increase within a frequency band including the resonant frequencies of the structural system of the engine (a frequency band having a certain width due to combined resonances of certain components located on a substantial transmission path of the engine). Hereinafter, such a frequency band relating to the resonant frequency is referred to as the “resonant frequency band.” Although a structural system of an engine generally has a plurality of resonant frequency bands, with the art of JP3803903B2 described above, only the knocking noise within the specific frequency band of 2.8 to 3.5 kHz can be reduced, and the knocking noise occurring within the plurality of resonant frequency bands of the structural system of the engine cannot suitably be reduced.
Meanwhile, the knocking noise has characteristics corresponding to, in addition to the resonant frequencies of the structural system of the engine described above, an in-cylinder pressure level corresponding to a combustion excitation force. The in-cylinder pressure level is commonly referred to as “CPL (Cylinder Pressure Level)” and indicates high frequency energy obtained by Fourier transforming an in-cylinder pressure waveform with respect to a combustion excitation force index. Hereinafter, the in-cylinder pressure level is simply referred to as the “CPL.” The CPL corresponds to a heat generation rate indicating a combustion mode inside the cylinder. According to experiments conducted by the present inventors, it was found that a waveform of the heat generation rate changes under influences of environmental conditions, such as temperature and pressure, and the knocking noise receives an influence of such change in the waveform of the heat generation rate. Therefore, the present inventors came to establish an idea that in order to suitably reduce the knocking noise, it is preferable to set a time interval of a plurality of fuel injections based on a timing at which a highest value (peak) of the heat generation rate taking into consideration the influences from the environmental conditions (e.g., the temperature and pressure) is obtained. With the art of JP3803903B2 described above, since the time interval of the plurality of fuel injections is controlled based on a timing at which the combustion pressure wave rises (corresponding to a timing at which the heat generation rate starts to increase), the reduction of the knocking noise is not sufficient.